Amplifier



Dec. 22, 1942. w. slx- AMPLIFIER Filed May l, 1940 2 sheets-sheet 1 Dec. 22, 1942. w. slx 2,305,743

AMPLIFIER Filed May 1, 1940 2 Sheets-Sheet ,2

?.femed Dee. 22, 1942 UNITED STATES PATENT OFFICE AMPLIFIER Willem Six, Eindhoven, Netherlands, assigner, by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Coma,

as Trustee Application May 1, 1940, Serial No. 332,819 In Germany April 2,2, 1939 3 Claims.

` be considered as approximately ohmic resistances so that matching is obtained if the transformation ratio of each of these corresponds to the square root of the ratio of the resistances to be matched. In this case the impedance of the amplifier as measured between the terminals of the primary winding of the input transformer,

ror between the terminals of the secondary winding of the output transformer, constitutes an ohmic resistance corresponding to the impedance of the transmission line connected thereto. However, this holds good only for those frequencies at which the leakage inductance and the the low-pass filter constituted by the input transformer or output transformer is, according to a further feature of the invention, proportioned that the cut-off frequency is higher than the highest frequency to be amplified by the amplifier, with the result that the frequency-range to be amplified coincides with that part ofA the transmission band of the low-pass filter in'which the image impedance, under'the conditions to be satisfied, can be considered as constant.

An amplifier which has an output transformer transformed in the above manner into a lowpass filter is particularly useful in connection with a negative feed-back by which, as is well known, non-linear distortion in the amplifier can be avoided. When, in a degeneratively backcoupled amplifier, the negative feed-back voltage is taken from the output circuit of an amplifier comprising an output transformer, phase diswinding capacities of the transformer may be neglected. At higher frequencies the input and output impedances of the amplifier can no longer be considered as ohmic.

The main object of my invention is to provide means by which the influence` of the leakageinductance and of the winding capacity of the input transformer or the output transformer, or both transformers of amplifiers, can be removed in the frequency-range to be transmitted, so that in this frequency-range the'input impedance or output impedance or both impedances of the amplifier always conserves an ohmic character.

In accordance with the invention I obtain the above object by so proportioning the capacities located in parallel with the winding of the input transformer, the output transformer or both transformers, that thesecapacities and the leakage self-lnductance of each transformer constitute a low-pass filter of ther-type whose cut-off frequency corresponds at least to the highest frequency to be amplified.

As is well known, a low-pass filter of the v,r-type has an image .impedance in the transmission range which is an ohmic resistance. This resistance `is substantially constant in a considerable 'part of that range, but increases in the vicinity of the cut-off frequency. To achieve correct matching in the frequency-range to be amplified placements occur at the higher frequencies to be amplified .due to the leakage inductance and the winding capacity of the output transformer. This phase displacement may attain such a high value that the negative feed-back passes into back-coupling which may cause self-oscillation of the amplifier. At the same time thevalue of the negative feed-back will vary-also at the higher frequencies. When utilizing negative feedback of the output circuit of. an amplier comprising anoutput transformer transformed into a low ,pass filter, and assuming the amplifier load to be substantially resistive, both the phase and the value of the negative feed-back remain substantially constant through the entire frequency range to be amplified so that there is no danger of self-oscillation of the amplifier in this range. As regards frequencies exceeding the limiting frequency of the low-pass lter, the phase of the l negative feed-back will of course vary, but also describe the same in more detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of e. line amplifler,

`Fig. 2 is a schematic diagram of the line amplifier of Fig. 1 with equivalent circuits shown for the transformers,

Fig. 3 is a vschematic diagram of the line amplifier of Fig. 1 with equivalent circuits of the transformers at the higher frequencies,

Fig. 4 is a graph showing the relationship between imag'e impedance and frequency,

Fig. 5 is a schematic diagram of an. amplifier with negative feed-back and embodying the invention,

Fig. 6 is a schematic diagram of an output circuit of an amplifier according to the invention,

Fig. 7 is a schematic diagram of an output circuit of an amplifier according to the invention and combining voltage feed-back and current feed-back,

Fig. 8 is an equivalent circuit for the ampilier of Fig. 7, and

Fig. 9 is a phase diagram of the circuit of Fig. 8.

VThe line amplifier shown in Figure 1 is connected between two transmission lines I and 6 and comprises a first amplifying tube i and an output tube 2 which has an internal resistance R2. One or more amplifying tubes (indicated by the reference numeral 50) may be connected between theamplifying stages represented by tubes i and 2. The amplifying tube I has an input circuit including an input transformer 3, which has a transformation ratio 1m, of such a value that the grid-cathode impedance of the tube I, which is determined by a resistance R1, is matched to the impedance of transmission line 4 whose impedance may be represented approximately by an ohmic resistance R.

As the resistances R1 and R2 are large relatively to the line impedances R; the input transformer 3 has to step up and the output transformer 5 has to step down. Owing totheir large number of turns the secondary winding of the input transformer 3 and the primary winding of the output transformer 5 lhave self -capacities which cannot be neglected. YMoreover, the grid-cathode capacity and anode-cathode capacity respectively of the amplifying tubes l and 2 respectively is connected in parallel with them. In Figure 1 the sum of these capacities are designated in dotted lines by condensers C1 and C2.

According to the invention condensers having values of (F1/nzl and C11/m2 are connected in parallel with the primary winding of the input transformer and in parallel with the secondary winding of the output transformer respectively.

As will appear from the following discussion, the input transformer 3 and the output transformer 5 are thus transformed into low-pass filters. When the input and output-transformers of the circuit shown in Fig. 1 are replaced by the equivalent circuit well known for transformers a circuit such as shown in Fig. 2 is obtained. In Fig. 2 the reference letters L1 and Le represent the primary leakage inductance and the secondary leakage inductance respectively, M1 represents the mutual inductance and n1 the transformation ratio of the input transformer 3. The reference letters La and L4 indicate the primary leakage inductance and the secondary leakage inductance respectively, Mz indicates the mutual. inductance and n2 indicates the transformation ratio of the output transformer 5.

From the diagram shown in Fig. 2 it appears that at high frequencies the inductances L1, L2

and L3 and L4, as well as the winding capacities C1 and C2 have an influence and that at such frequencies the input impedance and output impedance of the amplifier are no `longer determined exclusively by the resistances R1 and R2. At high frequencies the impedance of the mutual inductances M1 and Mz has such a high value that they are negligible so that the diagram shown in Fig. 2 can be replaced by the diagram shown in Fig. 3 in which L:71.12L1+L2 and L"=L3+n22L4. As shown in Fig. l, a condenser is connected in parallel with the primary winding of the input-transformer 3, a capacity C1 is located between the input-terminals E1 and E2 in the diagram shown in Fig. 3 so that a low-passfilter ensues. The leakage inductances L1 and L2 and the secondary winding capacity C1, (which can be increased at will by the parallel-connection of a condenser) are proportioned so as to satisfy the relation:

In this case the image impedance Z,r of the lowpass filter may be represented -by the expression:

\/1(t in which f represents the frequency and fo equals and represents the cut-off frequency. R is the nominal value of the image impedance which corresponds to the impedance of the transmission line l.

Fig.4 shows the image impedance Z.r as a function of the frequency f. and from the curve thereof it appears that the image impedance, in a very broad frequency-range below the cut-of! frequency fo, is approximately constant with a nomnal value R and then increases with the frequency. When selecting a cut-off frequency fo which is higher than the highest frequency to be amplified by the amplifier in such manner that the frequency-range to be amplified coincides with the frequency-range in which the image impedance of the low-pass filter may be considered to be substantially constant, it is possible to obtain a sufficiently exact matching of the input-impedance of the amplifier to the transmission line 4 in the frequency-range to be amplified and consequently reflection phenomena are avoided.

In exactly the above manner a condenser can be connected in parallel with the secondary winding of the output transformer 5 so that a capacity C2 lies 'between the output terminals U1 and U2 in the diagram shown in Fig. 3, and that a low-pass filter is formed also'in the output circuit of the amplifier. This filter is likewise proportioned so that the output impedance of the amplifier in the frequency range to be amplified is matched to the impedance R of the transmission line 6.

After transforming the output transformer of an amplifier into a low-pass filter in the above manner the 'use of negative feedback extending from the output circuit to the input circuit of a preceding amplifying tube has special advantages. This will be pointed out with reference to Fig. 5 which shows an amplifier with voltage feedback and comprises two stages including amplifying tubes I and 2 interconnected by a network 1. The primary winding of the input transformer 3 and the secondary winding of the output transformer 5 are shunted by condensers C1 and Cz respectively,

so that they are transformed into low-pass filters, and the input impedance and output impedance of the amplifier are matched to the impedance of A.

the transmission line 4 and 5 respectively. Volt-y age feedback of the output circuit of the amplifying tube 2 on to the input-circuit of the amplifying tube I is achievedby means of a third winding 8 of the output-transformer 5, which winding is connected in the grid circuit of the tube l. As has been set forth above, the impedance, `measured between the primary terminals ofthe output-transformer in the frequency-range tobe amplified, is av` constant resistance, and therefore the value and phase of the degeneratively back-coupled voltage across thewinding 8 and the output-transformer are also constant. Consequently, undesirable phase-displacements of the voltage fed back cannot occur in the frequency-range to be amplified.

For frequencies exceeding the cutoff frequency of the low-pass filter, the impedance measured between Vthe primary terminals of the output transformer is capacitative. Consequently the phaseangle between the voltage induced in winding 8 and the voltage to be amplified and supplied to tube I decreases with increasing frequency with the result that the negative feedback vcan passinto a back-coupling at sufficiently high frequencies. However, from the curve shown in Fig. 4 it appears that the value of the'impedance Zw, and consequently also the voltage between the primary terminals of the output transformer 5, rapidly decreases above the cut-off frequency of the low-pass filter, so that the stated decrease of the' phase angle results in a decrease in value of the generatively back-coupled voltage. Consequently, for frequencies which are so high that back-coupling instead of negative feedback occurs, self-oscillation of the amplifier cannot occur in spite thereof, because the back-coupled voltage is too small at these frequencies.

In some cases it may be desirable that the amplitude of the back-coupled voltage induced in the winding 8 be maintained constant in the entire transmission band of the low-pass filter constituted by the output-transformer. This can be ensured by giving the capacity connected in parallel with the primary winding a'larger value, preferably equal to 2Cz. In this case the outputcircuit of the amplifying tube v2 is connected in the manner shown in Fig. 6 in which the same reference characters Vare used to indicate the same parts of Fig. 5.

The impedance Z measured between the primary terminals of the output-transformer 5 is now constituted by the image impedance Z1r and the impedance of a condenser Cz connected in parallel therewith.

Zr 1 -i-jwCzZ, The absolute value of Z, i. e. \Zl,equals:

l f0=2fRC2 substitution of this value in the above term gives the following: i

Consequently the absolute value of the impedance Z is constant in the entire range.

In this case there is set up across the transmission primary winding in the transmission range a constant voltage which induces a voltage which is also constant. In this case the degenerative back-coupling always retains the same value.

The voltage feedback may advantageously be combined with a current feedback, and this will be explained with reference to Fig. 7. In circuit shown in this figure the negative feedback consists of two components i. e. a voltage which is set up across the winding 8 and depends on the voltage set up across the primary winding of the output transformer 4, and a voltage which is set up across the resistance 9 and depends on the current traversing the primary winding.

The phase-variations of the entire degeneratively back-coupled voltage with respectto frequencies lying outside the frequency-range are such that the negative feed-back never can pass into a back-coupling, as will be appreciated from the equivalent circuit shown in Fig. 8 and the corresponding phase-diagram shown in Fig. 9.

In the equivalent circuit of Fig. 8 the amplifying tube 2 is replaced by a source of E. M. F.,

shown as 'a generator e, and an internal resistance Rz. The impedance measured between the terminals of the primary winding ofthe output-transformer is represented by the image impedance Z1r of the low-pass filter. For. sim plicity it is assumed that the voltage induced in the winding 8 corresponds to the voltage set up across the primary winding so that the entire degeneratively back-coupled voltage ei corresponds to the sum of the voltage across the impedance Z and the voltage drop across the resistance Ru which corresponds, to the resistance 9. Again it is assumed, for simplicity, that Rz 1Ro+Z1rL The voltage e1 ls then determined by the expression and is consequently proportional to the resultant impedance Zt=Ro+Z1r. This resultant impedance Zn is indicated in the diagram shown in Fig.

. manner and is not subject to phase displacements in the transmission-range. With regard to the frequencies which are greater than the cut-off frequency, the impedance Z1r is capacitative. The value of this capacitative impedance decreases with the frequency. Hence, when the frequencies exceed the cut-off frequency the phase-angle of the total impedance Zz abruptly varies by an angle 4 which decreases with an increasing frequency. The phase-angle of the degeneratively lback-coupled voltage varies in exactly the same manner. Owing to the presence of the resistance Ro, which establishes the current feedback, the angle i is always smaller than 90; consequently the phase angle of the degeneratively back-coupled voltage always remains smaller than but decreases with an increasing frequency to thereby prevent the negative feedback from passing into a back-coupling.

Although I have described my invention with reference to specific examples and applications I do not desire to be limited thereto because obvious modifications will present themselves to one skilled in this art.

What I claim is:

1. In an'amplier having an input circuit and an output circuit,'means to obtain a degeneratively back-coupled voltage from the output circuit, anoutput transformer in said output circuit and provided with primary and secondary windings, current feedback means controlled by the current traversing said primary winding, and capacities connected in parallel with said windings, said capacities and the leakage inductance of the transformer forming a low-pass filter of the r type having a cut-off frequency corresponding at least to the highest frequency to be amplied.

2. In an amplier having an input circuit and an output circuit, means to obtain a degeneratively back-coupled voltage from the output circuit, an output transformer in said output circuit having a one-toone transformer ratio and being provided with a primary Winding and a secondary winding, a capacity connected in parallel with the secondary winding, and a capacity connected in parallel with the primary winding and having a value about twice the value of the first capacity,. said capacities and the leakage inductance of the transformer forming a lowpass lter of the 1r type having a cut-olf frequency corresponding at least to the highest frequency to be amplified.

3. In an amplifier having an input circuit and an output circuit, means to obtain a degeneratively back-coupled voltage from the output circuit, an output transformer in said output circuit and having a one-to-one transformer ratio and being provided with a primary winding and a `secondary winding, current feed-back means controlled by the current traversing said primary winding, a capacity connected in parallel with the secondary Winding, and a capacity connected in parallel with the primary Winding and having a value about twice the value of the first capacity, said capacities and the leakage inductance of the transformer forming a low-pass filter of the 1.- type having a cut-olf frequency corresponding at least to the highest frequency to be amplified.

WILLEM SIX. 

